Dehydrogenation of alkyl aromatic hydrocarbons, such as ethyl benzene to styrene, ethyl toluene to vinyl toluene, diethyl benzene to a mixture of divinyl benzene and vinyl ethyl benzene, isopropyl benzene to isopropenyl benzene, and ethyl naphthalene to vinyl naphthalene, by passing a mixture of steam and such alkyl aromatic hydrocarbon over a "self-regenerative" catalyst is known. The self-regenerative catalysts are usually described as those containing one or more oxides of iron, zinc, chromium or magnesium, as the major ingredient, and an alkali metal oxide, hydroxide or carbonate, particularly potassium or rubidium oxides, hydroxides or carbonates, as water gas reaction promoting ingredients. This reaction tends to mitigate carbon build-up on the catalyst surface, thereby permitting long periods of continuous dehydrogenation cycles for converting alkyl aromatic hydrocarbons having at least one alkyl group of 2 to 3 C atoms to the corresponding alkenyl aromatic hydrocarbon. Representative self-regenerative catalysts are disclosed in U.S. Pat. Nos. 2,370,797, 2,395,875, 2,414,585, 2,426,829, 2,461,147, 3,205,179 and 3,703,593. These catalysts usually contain one or more of the oxides of iron, zinc, chromium or magnesium as the major ingredient and an alkali metal oxide, hydroxide or carbonate, preferably the potassium compound, as a water gas reaction promoting ingredient. The catalysts may also contain other additives such as stabilizers, binders and porosity control agents. These additives are known in the art and are described in the above-cited patents.
In our U.S. Pat. No. 3,907,916 it is demonstrated that a self-regenerative catalyst which consists essentially of a major proportion of at least one oxide of iron, zinc, chromium or magnesium, and which also contains an amount of an alkali metal oxide, hydroxide, or carbonate, preferably the potassium compounds, to promote the water gas reaction, a chromium compound as a promoter, and, optionally, a stabilizer, binders and/or porosity control agents, can be activated by short, periodic steaming cycles (without presence of alkyl aromatic hydrocarbon). A steaming cycle of about 7-30 minutes every 24-48 hours at a temperature of from about 600.degree. C. to about 700.degree. C., is sufficient to activate the catalysts. Most conveniently the previous operating temperature is used for activation.
Activation of the catalyst apparently is not due to decoking, because little or no carbon oxides are found in the effluent during the activation cycle.
After activation, the conversion of alkyl aromatic hydrocarbons to alkenyl derivatives is increased, the selectivity remains high and the dehydrogenation cycle can be run under high severity conditions, e.g., low steam to hydrocarbon ratios or higher temperatures or both without excessive coking of the catalyst. It is thus possible to attain increased throughput per reactor and also to obtain higher yields of alkenyl aromatic hydrocarbon at lower reaction temperatures.
Although that activation process is highly effective as shown in the examples of U.S. Pat. No. 3,907,916, it has certain disadvantages when used as described in large scale plant operation. These disadvantages include pressure surges when the hydrocarbon feed is abruptly terminated and restarted, the relatively long time required to purge the whole apparatus, and the consequent difficulty of determining the actual catalyst purge time. In addition, it has been found necessary to substitute some steam condensate for the hydrocarbon feed during the short steam activation cycle to prevent excessive temperatures from developing in the heat exchange system.